Pathophysiological fractionation of stable nitrogen and carbon isotopes during periods of nutritional stress has the potential to bias investigations of ancient diets using such methods, particularly when no corroborating evidence of stress is apparent. More stable isotope analyses of human proxies for nutritional stress are therefore necessary to resolve this issue of diet vs. stress. We show here that individuals undergoing bariatric surgery are an ideal proxy for studying pathophysiological fractionation among humans. Using sequential samples of hair from two individuals, we construct an isotopic timeline demonstrating a 1‰ enrichment in δ15N values immediately following bariatric surgery, reflecting the catabolism of body tissues. Stable carbon isotopes show little systematic change with tissue catabolism.